Methods for reducing protrusions and within die thickness variations on plated thin film

ABSTRACT

Embodiments of the invention provide methods for electroplating a substrate that substantially reduce or eliminate protrusions and decrease WID thickness variations. The number of protrusions formed on the plating surface is highly dependent upon the electroplating current density. Embodiments of the invention vary the electroplating current waveform by implementing an initial current step sufficient to fill substrate features and a terminal current step sufficient to achieve the specified plating thickness while suppressing protrusions and within die thickness variations.

This is a Divisional application of Ser. No. 10/397,106 filed Mar. 25,2003, which is presently pending.

FIELD

Embodiments of the invention relate generally to electroplatingintegrated circuit substrates and, more particularly, methods forreducing protrusions and within die thickness variations on plated thinfilm.

BACKGROUND

During the manufacture of integrated circuits, a semiconductor wafer isdeposited with a conductive metal to provide interconnects between theintegrated components. Aluminum deposition may be used for this purpose.Copper has recently been found to offer distinct advantages overaluminum as a conductive plating for an integrated circuit substrate.Copper is more conductive than aluminum and can be plated into muchsmaller features (e.g., trenches and vias) having high aspect ratios.This is an important advantage given the trend toward smaller features.Moreover, the deposition process for aluminum is more costly andcomplex, requiring thermal processing within a vacuum, whereaselectroplating can be used to effect copper plating of semiconductorwafers.

The use of copper plating, however, is not without drawbacks. Twosignificant drawbacks are defects in the copper plating and within diethickness variation of the copper plating.

Defects

During electroplating and subsequent processing, a variety of critical(killer) and non-critical defects can be developed. Critical defectsinclude, for example, the copper plating being scraped by contaminantparticles introduced during handling or impurities in the electroplatingbath. During the chemical/mechanical planarization process, whichincludes polishing, particles on the copper plating may scrape or deformthe surface. If such scrapes or deformities are substantial, they candamage the die underneath and eventually impact final yield.

Another example of a critical defect is a crater in the copper platingcaused by a corrosive solution used in the fabrication process. Duringprocessing, the edge of a plated wafer is typically etched to allowmechanical handling. Etching is accomplished through use of a corrosivesolution that may inadvertently splash onto the region where copperetching is absolutely not desired. The unplated area of the wafer willbe destroyed in subsequent processing.

Substrates having critical defects in their copper plating will bediscarded, while processing continues for substrates with non-criticaldefects. It is important to be able to determine the critical defectdensity because critical defects can have a tremendous impact on yieldrates, so critical defects must be identified. This is done by reviewingthe plated surface with a scanning electron microscope (SEM). Typically,the non-critical defects outnumber the critical defects, and the vastnumber of non-critical defects make it very difficult to identify thecritical defects.

The most prevalent type of non-critical defect is known as a copperprotrusion, which is a copper bump, typically 20-50 nm in diameter and50-500 nm in length, extending from the plating surface. Theoverwhelming number of copper protrusions renders a 100% SEM reviewprohibitive, but a representative review is error-prone.

Within Die Thickness Variations

The other substantial drawback of copper plating is WID thicknessvariation. Prior to plating, the semiconductor wafer is patterned withvias and trenches that form the interconnects. With typical conformalelectroplating, the electroplate metal will grow at a similar rate overthe entire surface being plated. If the surface is not flat, the metalwill follow the contours of the surface. Conformal electroplating is notsuitable for surfaces having small features, as it tends to leave a seamor hole inside the feature at the end of the plating. FIG. 1Aillustrates the drawbacks of conformal electroplating for surfaceshaving small features in accordance with the prior art. As shown in FIG.1A, the substrate 100 has a number of features labeled 105A-105D thatmay be trenches or vias. Using conformal electroplating may cause holes,as shown in features 105A and 105C, or seams, as shown in features 105Band 105D, to form over the features. This problem is more pronounced forsmaller features.

To address the problem of seams and holes in the copper plating, asuppressant and accelerator are added to the electroplating bath tosuppress copper plating outside the features (in the field regions 115)while accelerating copper deposition at the bottom of the features. Theaccelerator allows the copper plating to grow faster from within thefeatures, filling the features from the bottom up to avoid the formationof holes and seams in the copper plating. The accelerator is known asbottom-up superfill or momentum electroplating. However, because thecopper plating continues to grow at a faster rate over the features evenafter filling the features, the “hump” is formed over the features,causing a WID thickness variation. This is known as momentumelectroplating. WID is the step height difference between the copperplating area over a feature region and the copper plating area over afield region. FIG. 1B illustrates WID thickness variations in the copperplating due to momentum electroplating in accordance with the prior art.As shown in FIG. 1B, substrate 120 has a number of features labeled125A-125D that may be trenches or vias. Using momentum electroplatingwhile avoiding holes and seams cause a WID thickness variation 135 overeach feature. WID thickness variations typically range from 100-200nm.

The growth rate of the copper plating is related to the electroplatingcharge distribution rate. Typically, prior art thin film electroplatingschemes include an initial current step and a number of intermediatecurrent steps with gradually increasing current.

FIG. 2 illustrates a typical electroplating current waveform 200 inaccordance with the prior art. The initial current step (a specifiedcurrent level for a specified time) 201 of electroplating currentwaveform 200 has a relatively low current and is used to fill thesmallest features. Larger regions, for which the copper growth rate isequivalent to the field region growth rate, may be only partially filledduring the initial current step. Intermediate current steps 202 and 203are used to fill larger features. After the features are filled, thecurrent is increased at current step 204 to more quickly achieve aspecified thickness for the copper plating. Use of a waveform of thisgeneral type results in copper protrusions as discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1A illustrates the drawbacks of conformal electroplating forsurfaces having small features in accordance with the prior art;

FIG. 1B illustrates WID thickness variations in the copper plating dueto momentum electroplating in accordance with the prior art;

FIG. 2 illustrates a typical electroplating current waveform inaccordance with the prior art;

FIG. 3 illustrates a substrate having a copper thin film electroplatedon its surface in accordance with one embodiment of the invention;

FIG. 4 illustrates an electroplating current waveform in accordance withone embodiment of the invention; and

FIG. 5 illustrates a process by which a substrate is electroplated inaccordance with one embodiment of the invention.

DETAILED DESCRIPTION Overview

Embodiments of the invention provide methods for electroplating asubstrate that substantially reduce or eliminate protrusions anddecrease WID thickness variations. The number of protrusions formed onthe plating surface is highly dependent upon the electroplating currentdensity. Embodiments of the invention vary the electroplating currentwaveform relative to prior art thin film electroplating schemes in sucha way as to substantially reduce or eliminate protrusions. For oneembodiment, one or more intermediate current steps of typical artschemes are eliminated and the terminal current step is prolonged.

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Moreover, inventive aspects lie in less than all features of a singledisclosed embodiment. Thus, the claims following the DetailedDescription are hereby expressly incorporated into this DetailedDescription, with each claim standing on its own as a separateembodiment of this invention.

As discussed above in reference to FIG. 2, an initial, relatively lowcurrent step is employed to fill the smallest substrate features. Therelatively low current is applied long enough to fill the features. Thecurrent step used to fill the features is dependent upon the size of thefeatures; smaller features require a lower initial current. The lowercurrent causes the growth of relatively large copper grains on thesubstrate surface. Moreover, certain grains are particularly oriented toresult in high-growth rate. That is, the lower current results inhigh-growth grains that grow much faster than the neighboring grains,resulting in protrusions.

The grain size of a plated metal film is generally a function ofnucleation rate or nuclei density and grain growth rate. If thenucleation rate is much faster and nuclei density is high relative tothe growth rate, the final deposit will often have fine grain size.Therefore, eliminating one or more intermediate electroplating currentsteps of the prior art scheme and using a terminal high current step oflonger duration, creates a new nuclei for copper growth on the substratesurface having relatively smaller and randomly-oriented grains. Thissuppresses the growth of the relative large copper grains resulting fromthe initial current step and thereby reduces or eliminates protrusions.The increased duration of the terminal current step also substantiallydecreases WID thickness variation.

The intermediate electroplating current steps of the prior art schemedid not produce copper grains sufficient to suppress the growth of therelatively large grains produced by the initial current step. This isbecause the intermediate steps produced grains that, although these maynot have been large enough to cause protrusions, were not sufficientlysmall and randomly oriented to suppress the growth of the relativelylarge grains.

FIG. 3 illustrates a substrate having a copper thin film electroplatedon its surface in accordance with one embodiment of the invention. Asshown in FIG. 3, substrate 301 has features 302 and 303 formed on itssurface. These features (for example, vias or trenches) may havesub-micron dimensions. In accordance with an embodiment of theinvention, an initial electroplating current step is selected to fillthe features 302 and 303. Electroplating using the selected initialcurrent step results in copper layer 304 having a number of relativelylarge, high-growth oriented grains, shown for example as grain 305.Eliminating one or more intermediate current steps of the prior art andusing a terminal current step of longer duration results in copper layer306 having relatively small, and more randomly-oriented grains 307. Suchelectroplating results in a decrease in the number of protrusions and areduction in WID thickness variation. The copper layers 304 and 306 arenot to scale, typically the feature filling current step or stepsaccount for only approximately 10-15 of the total thickness of thecopper plating.

Electroplating Current Waveform

FIG. 4 illustrates an electroplating current waveform in accordance withone embodiment of the invention. The current levels and duration foreach current step are exemplary and may be modified in accordance withalternative embodiments of the invention. Electroplating currentwaveform 400, shown in FIG. 4, includes an initial current step 401 andan intermediate step 402. In accordance with one embodiment of theinvention, the current waveform 400 eliminates at least one of theintermediate current steps (i.e., current step 203) and increases theduration of the terminal, high current level, current step. That is, asshown in FIG. 4, the terminal current step has a current level equal tothe terminal current step of the prior art, but has an increasedduration. As can be discerned by reference to FIG. 4, the electroplatingcurrent waveform 400 applies approximately the same total electriccharge as the prior art electroplating current waveform 200, thusresulting in a copper plate of approximately the same thickness.

Initial current step 401 has a current level of2.25 A and a duration of10 seconds. This is sufficient to fill the smallest features on thesubstrate, for example, features having a dimension of less than 0.1microns. As the trend toward smaller feature size continues, the initialcurrent step may vary in current level and duration.

Intermediate current step 402 has a current level of 6.75 A and aduration of 30 seconds. This is sufficient to fill intermediate sizefeatures that may not have been filled during the initial current step401.

The terminal current step 403 has a current level of 33.75 A and aduration of 49 seconds. This is sufficient to suppress the growth ofprotrusions due to the relatively large, high-growth oriented grainsresulting from current steps 401 and 402. The modification of thecurrent level values and/or duration of the initial or intermediatecurrent steps may require the modification of the terminal current step.

Process

FIG. 5 illustrates a process by which a substrate is electroplated inaccordance with one embodiment of the invention. Process 500, shown inFIG. 5, begins with operation 505 in which the substrate features areevaluated. The dimensions of the substrate features determine thecurrent level and the duration of the initial current step. For example,smaller features may require an optimized initial current level.Moreover, the feature dimensions determine not only the current leveland duration of an intermediate current step, but also whether anintermediate current step is necessary. For example, if all of thefeatures are within a given dimension range, no intermediate currentstep may be necessary; that is, the initial current step may besufficient to fill all of the features of the substrate.

At operation 510 the substrate is electroplated using the initialcurrent step, and intermediate step, if any, determined based upon thedimensions of the feature substrate. This electroplating may result inthe formation of relatively large, high-growth oriented grains. Also,low current level of an initial current step in order to fill smallfeatures cause increased WID thickness variation.

At operation 515 the terminal current step is determined based upon theinitial and intermediate current steps, and upon the specified overallplating thickness. That is, the current level that is sufficient tosubstantially reduce protrusions is dependent, not only on the relativethickness of the copper layer formed by the initial and intermediatecurrent steps, but also on the specified overall plating thickness. Forexample, for a 1.0 micron copper film, the current level of the terminalcurrent step has to be higher than approximately 15.75 A, whereas for a0.5 micron plating the current level of the terminal current step has tobe higher than approximately 6.25 A. In general, for variousembodiments, a waveform can significantly reduce total protrusiondefects regardless of the initial current and total thickness if acharacteristic parameter, 1/{overscore (L)}, defined as$\frac{1}{\overset{\_}{L}} = {\frac{t_{gapfill}}{t_{overplating}}\frac{1}{t_{total}}}$is less than 0.4; where t_(gapfill) is the thickness plated with theinitial and intermediate current steps to fill the features,t_(overplating) is the thickness plated with the terminal current step,and t_(total) (≦1) is the total thickness of the copper film.

At operation 520 the substrate is electroplated using the terminalcurrent step. The resulting plating has substantially less protrusiondefects and substantially reduced WID thickness variations in relationto prior art electroplating schemes.

General Matters

Embodiments of the invention provide methods for electroplating asubstrate that substantially reduce or eliminate protrusions anddecrease WID thickness variations.

In reference to FIG. 4, an electroplating current waveform for anembodiment was described that included an intermediate current stepsufficient to fill intermediate size features that may not have beenfilled during the initial current step. In alternative embodiments, forexample that do not include intermediate size features, or in which theinitial current step is sufficient to fill any such intermediatefeatures, the intermediate current step may be omitted. Further, theterminal current step may be modified in consideration of the initialand intermediate current steps and in consideration of the specifiedplating thickness. For example, if a thicker plating was specified, theduration of the terminal current step may be increased. Likewise, if thecurrent level of the initial current step was decreased, for example tofill a smaller feature size, then the terminal current step may bemodified accordingly.

In reference to FIG. 5, the initial current step is determined so as tofill the substrate features. However, this can lead to increased WIDthickness variation. For one embodiment, the terminal current step isdetermined so as to substantially reduce the resulting WID thicknessvariations.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting.

1-9. (canceled)
 10. An apparatus comprising: a substrate having one ormore features formed thereon; and a layer of conductive metal formed onthe substrate by electroplating the substrate using an electroplatingcurrent waveform having an initial current step that causes the one ormore features to be filled with the conductive metal, and a terminalcurrent step that suppresses the formation of protrusions of theconductive metal.
 11. The apparatus of claim 10 wherein the substrate issilicon and the conductive metal is a metal selected from the groupconsisting essentially of copper, silver, gold and alloys thereof. 12.The apparatus of claim 10 wherein at least one of the plurality offeatures has a sub-micron dimension and a high aspect ratio.
 13. Theapparatus of claim 10 wherein the initial current step is sufficient tofill the plurality of features with the conductive metal.
 14. Theapparatus of claim 10 wherein the initial current step is not sufficientto fill the plurality of features with the conductive metal and whereinthe electroplating current waveform has an intermediate current stepthat is sufficient to fill the plurality of features with the conductivemetal.
 15. The apparatus of claim 14 wherein a total thickness of theconductive metal electroplated onto the substrate is approximately 1.0micron.
 16. The apparatus of claim 15 wherein the terminal current stephas a current level that is higher than approximately 15.75 A.
 17. Theapparatus of claim 14 wherein a total thickness of the conductive metalelectroplated onto the substrate is approximately 0.5 microns.
 18. Theapparatus of claim 17 wherein the terminal current step has a currentlevel that is higher than approximately 6.25 A.
 19. The apparatus ofclaim 10 wherein electroplating the substrates using the terminalcurrent step reduces the within die thickness variation of the layer ofconductive metal formed on the substrate. 20-26. (canceled)
 27. Anelectroplating waveform for electroplating conductive metal film on asubstrate comprising; an initial current step to fill a plurality offeatures formed within the substrate; and a terminal current step havinga current level and duration that suppresses the formation ofprotrusions of the conductive metal from the conductive metal film. 28.The electroplating waveform of claim 27 wherein the terminal currentstep results in a reduction of within die thickness variations.
 29. Theelectroplating waveform of claim 28 wherein the initial current step isinsufficient to fill the plurality of features further comprising: anintermediate current step to fill any of the plurality of features notfilled by the initial current step.
 30. The electroplating waveform ofclaim 29 wherein a ratio of a portion of the conductive metal filmformed by the initial current step and the intermediate current step toa portion of the conductive metal film formed by the terminal currentstep, multiplied by a reciprocal of the total thickness of theconductive metal film is less than 0.4.